Liposomes are small vesicles composed of amphipathic lipids arranged in spherical bilayers. Liposomes are usually classified as small unilamellar vesicles (SUV), large unilamellar vesicles (LUV), or multi-lamellar vesicles (MLV). SUVs and LUVs, by definition, have only one bilayer, whereas MLVs contain many concentric bilayers. Liposomes may be used to encapsulate various materials, by trapping hydrophilic compounds in the aqueous interior or between bilayers, or by trapping hydrophobic compounds within the bilayer.
Liposomes exhibit a wide variety of characteristics, depending upon their size, composition, and charge. For example, liposomes having a small percentage of unsaturated lipids tend to be slightly more permeable, while liposomes incorporating cholesterol or other sterols tend to be more rigid and less permeable. Liposomes may be positive, negative, or neutral in charge, depending on the hydrophilic group. For example, choline-based lipids impart a positive charge, phosphate- and sulfate-based lipids contribute a negative charge, and glycerol-based lipids and sterols are generally neutral in solution.
Liposomes have been employed to deliver biologically active material. See for example Allison, U.S. Pat. No. 4,053,585, which disclosed the administration of several antigens in negatively charged liposomes, optionally including killed M. tuberculosis. Fullerton et al., U.S. Pat. No. 4,261,975, disclosed the use of separated influenza membranes, with hemagglutinin spikes attached, which is bound to liposomes for use in influenza vaccines.
Liposomes have been used to encapsulate a large variety of compounds which exhibit poor aqueous solubility, or which exhibit unacceptable toxicity at therapeutic dosages. For example, amphotericin B is an antifungal antibiotic which is poorly soluble in water, alcohols, chloroform, and other common halocarbon solvents. While amphotericin B is an effective fungicide, it is also dangerously toxic at concentrations slightly above the therapeutic concentration. Encapsulation in liposomes appears to reduce the in vivo toxicity to mammalian cells, while leaving the fungicidal activity relatively unaltered (F. C. Szoka et al., Antimicrob Agents Chemother (1987) 31:421-29). The effects on cytotoxicity and fungicidal activity were dependent upon the particular liposome composition, liposomal structure (e.g., SUV, MLV, etc.), and method of preparation.
Phospholipid vesicles (liposomes) can be formed by a variety of techniques that, in general, start with "dry" lipids that are introduced into an aqueous phase (D. Lasic, J Theor Biol (1987) 124:35-41). Once the lipid is hydrated, liposomes form spontaneously. Techniques have been developed to control the number of lamellae in the liposomes and to produce a defined particle size. The available procedures are satisfactory for most applications where small amounts of material are needed (G. Gregoriadis, "Liposome Technology" I-III (Boca Raton, Fla., CRC Press, Inc.), 1984). However, for the manufacture of vesicles on a large scale, the lipid hydration step can be a severe constraint on vesicle production.
To accelerate the lipid hydration step, the lipids can be dissolved in an organic solvent and injected into the aqueous phase. This permits a continuous production of vesicles since the solvent can be removed by dialysis or evaporation. Using ethanol as the solvent, unilamellar liposomes of defined size can be formed by injection (S. Batzri et al., Biochem Biophys Acta (1973) 298:1015-1019; J. Kremer et al., Biochemistry (1977) 16:3932-3935). This procedure generates unilamellar vesicles as long as the lipid concentration in the ethanol is below 40 mM and the final ethanol concentration in the aqueous suspension is less than about 10% (F. Boller et al., EPO 87306202.0, filed 14 Jul. 1987). These two factors limit the concentration of defined sized liposomes formed by ethanol injection to about 4 mM. This is a rather dilute solution of liposomes; for water soluble compounds the encapsulation efficiency is poor, while for lipid soluble compounds large volumes are required to obtain a sufficient quantity of material. Because of these limitations, ethanol injection has not been widely employed for making lipid vesicles (D. Lichtenberg, et al., in "Methods of Biochemical Analysis" (D. Glick, ed., John Wiley & Sons, N.Y. (1988)), 35:337-462).
Lipid particles are complexes of an amphipathic lipid with another molecule, in a defined ratio, which result in a supramolecular structure or particle. The principal difference between a liposome and a lipidic particle is that a liposome has a continuous bilayer of lipid surrounding an aqueous core, whereas a lipidic particle does not. Because of this difference, in most cases, lipidic particles cannot encapsulate water soluble molecules. Lipidic particles can range in size from about 5 nm to greater than 1,000 nm. The size of the final lipidic particle depends upon the composition and the method of preparation. Examples of lipidic particles are the lipid emulsions (S. Ljungberg et al., Acta Pharmaceutica Suecica (1970) 7:435-40), lipoproteins (A. Gotto et al., Meth Enzymol (1986) 129:783-89), and iscoms (K. Lovgren et al., J Immunol Meth (1987) 98:137-43).